1. Field of the Invention
This invention relates to the production of hydrocarbons from coal and is particularly concerned with coal liquefaction operations in which the liquefaction bottoms are pyrolyzed to produce additional products.
2. Description of the Prior Art
There have been numerous processes developed for the production of liquid hydrocarbons from coal and similar carbonaceous solids. Among the most promising of these are processes in which the feed coal is first contacted with a hydrogen-containing gas and a hydrogen-donor solvent at elevated temperature and pressure in a liquefaction reactor and a portion of the liquid product is then catalytically hydrogenated in a solvent hydrogenation reactor to generate additional liquid products and solvent for recycle to the liquefaction step. If the liquefaction and solvent hydrogenation steps are carried out under similar pressure conditions, the vaporous products formed during liquefaction may be passed directly to the solvent hydrogenation reactor. Otherwise, these products will generally be treated for the removal of contaminants, compressed and heated, and then introduced into the solvent hydrogenation vessel. The liquid effluent from the liquefaction step is normally passed to a low pressure separator in which gases are taken off and then fractionated, the lighter constituents being employed as feed to the solvent hydrogenation reactor and the heavier material which for the most part boils above about 1000.degree. F. and contains ash and unreacted coil solids being recovered for upgrading into lower boiling products. Alternatively, the liquid stream obtained following the removal of gases from the liquefaction reactor liquid effluent can be mixed with a hydrocarbon liquid or antisolvent and then treated for the removal of solids by settling, filtration or centrifugation. The liquid overhead fraction recovered from this solids removal step can then be fractionated to produce liquids suitable for use in the hydrogenation step and a heavy bottoms fraction which can be further treated to produce lighter, lower boiling products. The liquid products obtained from the solvent hydrogenation step of the process are fractionated, the lighter constituents being taken overhead for use as fuel or the like and the heavier constituents being recycled for use as solvent in the liquefaction step or recovered as additional product.
One disadvantage of processes of the type described above and other coal liquefaction systems is that the amount of heavy bottoms produced during liquefaction is generally high and may constitute as much as half of the total yield from the liquefaction step. There have been numerous proposals for converting these heavy bottoms into lighter products of higher value, including suggestions that they be used as feed for hydrogenation, catalytic cracking, or pyrolysis operations. The bottoms are normally difficult to hydrogenate and are generally poor candidates for catalytic cracking because contaminants present may damage the cracking catalyst. Pyrolysis in a batch or continuous coking unit is therefore considered the most promising method for upgrading liquefaction bottoms. This results in the production of additional liquids and gases and in the formation of coke which can be subsequently gasified to produce hydrogen useful in the liquefaction operation. The gas and liquid yields from the pyrolysis step tend to be low and the amount of coke produced tends to be high. Efforts to improve the yields by reducing coke formation during pyrolysis have met with only limited success.